This invention relates generally to injection molding and more particularly to a heated nozzle which receives less heat along a portion adjacent its rear end and a method of making it.
Elongated injection molding nozzles having an electrical heating element integrally brazed in a spiral channel extending around a central melt passage with two ends extending to an external terminal to receive a pair of external power leads are well known in the art. An example is shown in the applicants' Canadian patent application serial number 2,057,594-8 filed Dec. 13, 1991 entitled "Method of Making an Injection Molding Nozzle With a Single Heating Element in a Double Channel." The heating element has a central wire with a fine coiled heating portion extending between two conductive portions. The heating portion of the central wire has a small diameter with sufficient resistance to generate considerable heat from the current flowing through it, while the two conductive portions have large enough diameters that very little heat is generated by the current. In these previous nozzles the heating portion of the central wire is in the portion of the heating element extending in the spiral channel around the melt passage, while the two conductive portions of the central wire are only in equal length portions of the heating element extending outwardly from the spiral channel to the external terminal. Thus heat is provided by the heating portion of the central wire along the melt passage, but little or no heat is provided by the conductive portions of the central wire adjacent the external terminal.
In order to maintain the temperature of the melt flowing through the melt passage within a narrow temperature window, it is known that less heat needs to be provided in areas where there is less heat loss. Thus, as mentioned in the applicants' U.S. Pat. No. 4,557,685 which issued Dec. 10, 1985, it is known to provide the spiral channel in which the heating element is wound with a varying pitch, depending upon heating requirements. While this works very well for many applications, it is relatively costly to make and a nozzle with a channel having a particular pitch must be used only for the particular application it was designed for.